Showing papers by "Patrick S. Nicholson published in 1988"

The mechanical and electrical properties of ZrO2-Naβ″-Al2O3 composites

Abstract: ZrO2-Naβ″-Al2O3 composites were prepared by a conventional method using two different powder routes and different milling liquids. The retained tetragonal-phase ZrO2 was 85 to 90% for composites with 2.4 to 15 vol% ZrO2. The fracture toughness (Klc) and strength increased with increasing ZrO2 content. At 20 vol % ZrO2,Klc and bend strength were 4.35 M Pa m1/2 and 390 MPa, respectively. Stress-induced transformation toughening is the predominant toughening mechanism. Dispersion toughening also contributes to the increase ofKlc. Surface strengthening was found to be an effective strengthening method for low ZrO2 levels. The critical tetragonal ZrO2 grain size was found to increase from 0.86 to 1.02gmm as the ZrO2 content increased from 2.5 to 15 vol %. A detailed study of the ionic conductivity of the 15 vol % ZrO2 dispersed sample was conducted by an a.c. technique between −124° C and ∼ 300° C. The bulk and total conductivities were calculated via complex-plane analysis. The total (grain and grain-boundary) ionic specific resistivity was ∼ 9 Ω cm at 300° C. The activation enthalpies of the bulk and total conductivity processes were 0.30 and 0.32 eV, respectively.

Strength Improvement of Yttria-Partially-Stabilized Zirconia by Flaw Elimination

Abstract: A systematic improvement of the flexural strength of conventionally sintered 4.5-wt%-Y2O3-partially-stabilized zirconia was realized by identifying the fracture origins of four-point-bend bars and eliminating them by process modification. The average flexural strength increased from 880 to 1380 MPa, a value comparable with that of isostatically hot-pressed or slip-cast material. A part-through crack extension model was used to correlate the fracture stresses and the fracture origin sizes and a “fracture origin severity parameter” was identified.

Inclusion‐Size‐Independent Strength of Glass/Particulate‐Metal Composites

Abstract: Dispersed-phase metallic inclusions limit brittie-matrix defect size and contribute to toughness via plastic deformation. In this work, it is shown that composite strength depends on the dispersed-phase volume fraction and not on the diameter of uniformly sized inclusions if the plastic deformation thereof is fully utilized. Experimental results for silicate glass/aluminum-particle systems substantiate this finding. Particulate composites of 40 vol% Al and 40 vol% Al-alloy soda–lime–silica glass exhibit 1.5- and 2-fold strength increases, respectively. Both 20 vol% Al composites with 57- and 20-μm particle size showed 1- to 3-fold increases in strength, respectively. Model calculations of strength are overestimates and it is proposed that particle/matrix interfacial separation limits full utilization of the potential plastic work of deformation.

Microstructural development of a ZrO2 Naβ″-Al2O3 composite

Abstract: Microstructural development in Naβ″-Al2O3 containing 15 vol% ZrO2 particles is described. Intergranular ZrO2 particles inhibit abnormal grain growth of the Naβ″-Al2O3. The growth of the Naβ″-Al2O3 grains and the intergranular ZrO2 followed a cubic time law. Direct particle coalescence consisting of encounter and spheroidizing processes was found to be the basic growth mechanism for the intergranular ZrO2

Surface strengthened composite ceramic material

Abstract: A surface-strengthened composite ceramic material has a ceramic matrix and a refractory phase dispersed at least in and close to the surface of the matrix. The refractory phase includes beta-alumina particles in which larger cations producing a larger molar volume replace sufficient smaller cations in beta-alumina particles in and close to the surface of the composite ceramic material to cause compressive surface stresses which increase the surface strength of the composite ceramic material. The smaller cations are replaced by the larger cations after firing of the composite ceramic material.

Method of making a focussing element for use in a lenseless focussed transducer

Abstract: A focussing element for use in a lenseless, focussed transducer for producing a semi-focussed, ultrasonic beam comprises a body formed of electrical insulating material and a plurality of piezoelectric elements disposed in one end of the body, each element being planar, extending outwardly from a common axis, and being disposed at a predetermined angel to the common axis whereby to define a common focal point on the axis, and electrical conductors secured to the piezoelectric element for electrically exciting the elements.

Structural Design and Synthesis of H3O+-β”/βAl2O3 and NH4-β”/β-Al2O3 Polycrystals

Abstract: The synthesis and characterization of H 3 Oβ”/β-Al 2 O 3 and NH 4 + - H 3 O+β”/β-Al 2 O 3 poly-crystals is described. The role of stress development and control in the intermediate stages of ion-exchange of the NaK-β”/βA1 2 O 3 ceramics is analysed and identifies the β-Al 2 O 3 phase as acting as inert reinforcement for the two-phase composite. The Rb + ion-exchange of the intermediate K + β”/β-Al 2 O 3 requisite for NH 4 +/H 3 O + ion-exchange to NH 4 -H 3 O+β”/βAl 2 O 3 is described as also is the NH 4 NO 3 exchange to the final ceramic. The total weight loss on heating NH 4 -H 3 O+β”/βAl 2 O 3 polycrystals to 810°C is 7.4% which agrees with the value expected from the chemical formula. The 25°C conductivity of the grains of NH 4 + - H 3 O + β”/βAl 2 O 3 was 1.7 × 10 −4 (ω-cm) −1 and the bulk conductivity was 3 × 10 −5 (ω-cm) −1 .

THE ANALYSIS OF THE FRACTURE ORIGIN TYPES AND SEVERITY IN Y2O3-PARTIALLY STABILIZED ZrO2

Abstract: A fractographic study was conducted on 45 wt% Y2O3-partially-stabilized ZrO2 four-point bend bars densified by dry-pressing/isostatic pressing/pressureless sintering Several fracture origin types were identified by SEM/EPMA and classified by their responses to various elimination techniques An elliptical crack model was used to characterize the correlation between fracture stress and fracture origin size Consistent results showed that an order-of-severity existed, which enables a definition of “fracture origin severity parameter” for the different origin types The existence of a relative fracture-origin severity is related to the residual stress fields at the defect-matrix interfaces A spherical-defect model yielded satisfactory predictions of the numerical values of the fracture origin severity parameters

Potential drop technique for following high-temperature crack growth in ceramics

Abstract: An electrical potential drop technique has been developed for the determination of curved crack front advance in conductive ceramics. The solutions obtained are applied to chevron-notched four-point bend specimens. Fracture of ionically conducting zirconium oxide ceramics is studied in the range 1000° to 1300°C. A driving force of the order of 1 J/m2 is sufficient to initiate fracture of the ceramic at High temperatures.

Electrical Relaxation Studies in Fluorite Oxides

Abstract: Electric relaxation in CeO2-M203 (M34 sY3+, La3+) solid solutions has been investigated as a function of temperature (373K-673K) using the electric modulus formalism in the frequency range 5 to 107Hz. Two relaxation processes are observed in dilute solid solutions. The low frequency process is identified as a long range migration of free oxygen-vacancies (Process A) and the high frequency process is due to reorientation relaxation of the (MceVo) charged associates (Process B). The relaxation process is analysed using a non-exponential decay function, o(t)=exp[-(t/τo)B] for O<β≤1, of the electric field. The observed activation enthalpy minimum as a function of dopant concentration for the Process A is explained using the concept of incomplete dissociation of oxygen-vacancies from (MceVo) defect associates and the formation of higher-order defect clusters at higher mole% M203.